Lung Volumes Flashcards
What are the indications for measuring lung volumes?
Differentiate between obstructive vs restrictive
Assess response to therapy
Aid in the interpretation of other lung function tests
Determine severity of disease
What are the 4 lung volumes?
IRV, ERV, Vt, RV
Define lung capacities and name them
Consist of two or more lung volumes together
FRC, TLC, IC, VC
What is the definition of a VC or SVC?
Maximum volume of air that can be exhaled from a maximum inhalation or vice versa
SVC=FRC (normally or withing 150 mls)
what are the two techniques for performing a SVC?
Maximum inhalation followed by a maximum exhalation (most common) (VCex)
Maximum exhalation followed by a maximum inhalation (VCin)
What biological factors influence VC?
Height, age, gender, race
What is the definition of Tidal Volume (Vt)?
Volume of of inhaled and exhaled with each breath
stays relatively normal (5-7ml/kg) regardless of any disease
What is the definition of an inspiratory capacity (IC)?
LArgest volume of air that can be inhaled from a resting exhalation
What is the definition of Expiratory Reserve Volume (ERV)?
The volume of gas that can be normally exhaled from a normal expiratory baseline
May be slightly reduced with obesity or neuromuscular problems
What is the definition of Functional Residual Capacity (FRC)?
The volume of gas remaining in the lungs at the end of normal expiration
FRC=End Expiratory Lung Volume (EELV)
What are the two techniques for measuring FRC?
- Plethysmography
2. Gas dilution methods: He dilution or N2 washout
Describe a pressure box
Constant volume box
Box is closed to atmosphere
Functions on Boyle’s law
Changes in box pressure/volume reflect Palv changes and Vthorax changes
Describe a flow box
Constant pressure box
Open to atmosphere
Flow is converted to volume
Pneumotach needs to be very sensitive to measure small flow changes
What are the uses of Body Plethysmographs?
Measure FRCpl, Raw, Cl and static lung volumes and spirometry
Flow measured via pressure differential pneumotach
Pbox and Pmouth via pressure transducers
Shutter attached to pneumotach occlude airway at FRC and measure Palv
What is Boyle’s law?
Volume of gas at a constant temperature varies inversely to the pressure applied to it
P1V1=P2V2
Describe the He dilution method
Known as closed circuit method
Easy for the patient to perform (requires only tidal breathing)
instruments used are simple and inexpensive
C1V1=C2V2
Patients FRC dilutes initial He concentration to a new concentration
The change in He concentration is directly proportional to the patients FRC
Describe the nitrogen washout method
Known as open circuit FRC method
Volume of N2 in the lungs start = volume of N2 in the spirometer at end
Patient breaths in 100% O2 to washout N2 (until less than 1.5% N2 in alveoli)
Totla volume of gas exhaled is measured and % of N2 in exhaled gas is measured
What happens to FRC in obstructive disorders?
FRC is greater than 120% and RV is greater than 120%
Also decreased flowrates
What happens to FRC in restrictive disorders?
FRC is decreased except in neuromuscular disorders
What is the definition of Residual Volume (RV)?
The volume in the lungs at the end of maximum exhalation
It is calculated once the FRC has been determined (increased RV means an increased work of breathing)
What can cause in increase in RV?
small airway obstruction
loss of elastic recoil
Loss of diaphragm function
loss of expiratory muscle force
What is the definition of Total Lung Capacity (TLC)?
Total volume in the lungs at the end of a maximal inspiration
Calculated value
Can me measured via CXR, MRI or CT
What is the significance of TLC?
Greater than 120% + other signs of obstruction = hyperinfaltion
Less than 120% + other signs of obstruction = airtrapping
Less than 80% = restrictive process
Describe RV/TLC%
Describes the % of TLC that must be ventilated by tidal breathing
Normal RV/TLC% = 20-35%
(Restrictive = less than 20, obstructive = increased value)
